Blender animations and Unity
Don't bother to do your animation work in Blender. Unity's animation tools are quite robust, and animating in Unity will make it easier to make different animations and behaviors for one model.

Blender behaviors like showing and hiding can't be translated accurately into Unity behaviors. So even if you had it working in Blender, you would have more work to do once you import it into Unity.

Just export what you have in Blender WITHOUT any animations and put it in Unity. Drop the model into your Unity scene, then open up the Animator. Click the dropdown under the record button and select [Create New Clip].



To show and hide your cube, go to the appropriate frame of the animation, then enable/disable its Mesh Renderer. This will automatically add a keyframe to the animation clip. Do the rest of your animation as you would normally, adding rotation keyframes and such.